1. Field of the Invention
This invention relates to a process for preparing a composition for a thermally developable light-sensitive material. More specifically, the invention relates to a process for preparing a thermally developable light-sensitive composition having a high sensitivity and a high contrast in which the size of the silver halide grains is controlled and the end point of the decomposition of the halogen atom-releasing compound can be accurately detected by reacting an organic silver salt with a halogen atom-releasing compound to form a silver halide in intimate contact with the surface of the organic silver salt while controlling the oxidation-reduction potential of the reaction solution.
2. Description of the Prior Art
The thermally developable light-sensitive materials described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075 are composed of a composition consisting essentially of an organic silver salt, a small amount of a silver halide and a reducing agent. Since they need only to be merely heated to at least 80.degree. C. after imagewise exposure to produce images, they are attracting attention as light-sensitive materials which can be processed completely in the dry state.
Of the methods for preparing a composition comprising a mixture of an organic silver salt, a small amount of a silver halide and a reducing agent for use in such a thermally developable light-sensitive material, the method described in U.S. Pat. No. 3,457,075 is superior to other methods because this method can be used to produce a silver halide which is in close proximity to the organic silver salt. This method comprises reacting a separately prepared organic silver salt with a small amount of a halogenating agent to convert a part of the organic silver salt into the corresponding silver halide and then adding a reducing agent to the mixture of the organic silver salt and the silver halide to form a composition for use in a thermally developable light-sensitive material (to be referred to hereinafter as a "halidizing method").
With this halidizing method, it is difficult to control the properties of the silver halide, for example, the silver halide grain size distribution, as desired. Since the starting materials, reaction solvents, materials such as a protective polymer, and other reaction conditions which are used in forming silver halide drastically differ from the materials and reaction conditions used for conventional gelatin-silver halide emulsions, the techniques and knowledge that have been built up in the field of producing gelatin-silver halide emulsions can only in certain instances be applied to thermally developable light-sensitive materials.
Further, various halogenating agents can be used in the above-described halidizing method. However, when a halogen atom-releasing compound (i.e., a compound capable of releasing a halogen atom or a radical containing a halogen when the compound is dissolved in a reaction medium and decomposition is accelerated such as by heating, etc.) other than a compound which completely dissociates in a reaction medium and releases a halogen ion (e.g., a metal halide or hydrogen halide) is used, whether or not the compound is completely decomposed must be confirmed. This is because if the decomposition of the compound is insufficient, the amount of a silver halide formed is relatively small and the compound which remains in the system without being decomposed has a function of cleaving spectrally sensitizing dyes for silver halide, which results in a reduction in the light-sensitivity of the thermally developable light-sensitive material. Further, it is also known that when the amount of the halogen atom-releasing compound remaining is large, a reduction of the contrast occurs in proportion thereto.
In order to prevent these drawbacks, in the conventionally employed halidizing method using a halogen atom-releasing compound since the halogen atom-releasing compound has the property of cleaving the spectrally sensitizing dye, a certain type of a merocyanine dye is added to the reaction solution, and sufficient decomposition of the halogen atom-releasing compound is confirmed by discoloration of the merocyanine dye. However, this is not a method for sufficiently and quantitatively confirming the decomposition of the halogen atom-releasing compound, and further, this method has various drawbacks that the results obtained vary depending upon the amount of the merocyanine dye added, and the structure of the merocyanine dye, and that an expensive merocyanine dye must be used. However, if such a halogen atom-releasing compound is merely decomposed to a sufficient extent, a thermally developable light-sensitive material which has a low degree of fogging and a high contrast as compared with a thermally developable light-sensitive material using a halogen ion-releasing compound can be obtained.